1. Field of the Invention
The present invention relates to a hybrid vehicle in which main driving wheels are driven by an engine and sub driving wheels are driven by a motor.
2. Description of the Related Art
A hybrid vehicle is described in JP-A-11-208304 in which main driving wheels are driven by an engine and/or a first motor and sub driving wheels are driven by a second motor. In the hybrid vehicle, a mode in which the vehicle is driven only by the motor and a mode in which the vehicle is driven by both the engine and the motor are combined in order to reduce its fuel consumption.
Incidentally, as is often the case with this kind of hybrid vehicle when attempting to impart the vehicle a running through performance that is to be provided by a four-wheel drive vehicle, the driving force of the rear wheels becomes insufficient. To be specific, this unfavorable fact becomes conspicuous {circle around (1)} when the vehicle runs on a gravelly road at low speed (in the order of 30 km/h) and {circle around (2)} when the vehicle climbs up an uphill road near a lakeside while hauling a boat.
In order to make the running under {circle around (1)} possible, the output of the motor for the rear wheels needs to be increased, and in order to make the running under {circle around (2)} possible, the torque of the motor for the rear wheels needs to be increased. In either of the cases, it is unavoidable that the size of the motor is enlarged. However, in case the motor is enlarged, there are caused some problems such as that the minimum ground clearance is reduced, an intersecting angle with the drive shafts is increased, and lowering the floor level of the vehicle becomes difficult.